1. Field of the Invention
This invention relates to a filter and blower unit for breathing masks or bonnets with a breathing air filter that is driven by a direct current motor and designed for use in potentially explosive areas.
2. Background Art
Breathing masks or bonnets supplied with air via a breathing air filter are commonly equipped with a filter and blower unit driven by a direct current motor to increase breathing comfort, that is, to make breathing through the filter material easier and facilitate long-term assignments of the wearer as well as the use of specific filter types.
Filter and blower units are often required to be designed for use in potentially explosive areas. This entails the requirement to design the direct current motor that drives the filter and blower unit in such a way that any sparks that may occur during its operation cannot ignite the potentially explosive atmosphere. Motors of filter and blower units that are no longer considered intrinsically safe due to their high rating and cannot be operated in potentially explosive atmospheres must therefore meet the requirements of the “d” type of protection—i.e. have an explosion-proof enclosure. An explosion-proof enclosure, however, is a disadvantage for filter and blower units as the manufacturing effort for motors of that design is very high due to the required narrow widths of gap and great gap lengths between the motor enclosure opening and the motor shaft, which makes such motors expensive. Blower motors designed according to the “d” type of protection are also quite heavy because of the measures required to neutralize the ignition power that can occur inside the motor enclosure. In addition, they cannot be used in the “O” zone.
Motors not designed according to the “d” type of protection can only be operated in an explosive atmosphere if power input including inductivity are not too high, i.e. still in the intrinsically safe range (“i” type of protection). The high-performance direct current motors used for filter and blower units do not meet this requirement.